1. Field of the Invention
The present invention relates to an optical communication device, and more particularly, to an optical fiber that can monitor or detect optical signals and an optical signal detecting apparatus using the optical fiber.
2. Description of the Related Art
In general, in order to monitor or detect optical signals propagating through an optical fiber core, the optical path of a core mode needs to be changed, and it can be changed by generating an optical fiber grating in the core. For example, when a tilted Bragg grating is formed in the core, a forward-propagating core mode can be changed to a backward-propagating cladding mode, and when a long period fiber grating is formed in the core, a forward-propagating core mode can be changed to a forward-propagating cladding mode.
Hereinafter, a conventional method of detecting optical signals and changing an optical path will be described briefly with reference to FIGS. 1 and 2.
FIG. 1 is a cross-sectional view illustrating an optical device in which optical paths are changed and detected using two optical fibers including tilted Bragg gratings, which is disclosed in U.S. Pat. No. 6,850,665. Hereinafter, the structure and the function of the optical device will be described briefly.
Referring to FIG. 1, the optical device for changing path and detecting optical signals is formed of two optical fibers 10 and 20 coupled together, wherein the two optical fibers 10 and 20 include tilted Bragg gratings 14 and 24 which are tilted in cores 12 and 22. Optical signals are detected as follows. An optical signal propagates through a core of an upper optical fiber 10 to the right and is coupled to a cladding mode by the tilted Bragg grating 14 and then propagates through the cladding 16 to the left. Then the optical signal is optically coupled by an optical fiber coupling portion 25 to propagate through a cladding 26 of a lower optical fiber 20 and then is optically coupled with a core mode by the tilted Bragg grating 24 and propagates to the right through the core 22. Although not illustrated in FIG. 1, the optical signal propagating through the core 22 is monitored or detected by an optical signal receiving device such as a monitor photodiode (mPD).
In the above described structure, it is necessary to have the sufficient optical coupling between two optical waveguides to monitor or detect optical signals delivered in the core mode, and in order to achieve sufficient optical coupling, the optical fiber coupling portions 25 should be up to several tens of cm. Accordingly, the size of the optical signal detecting apparatus becomes large and inappropriate for high integration.
Also, an optical device having an in-line structure which does not require an additional optical waveguide has been suggested. The optical device uses only one optical fiber in which a tilted Bragg grating is formed and thus can be manufactured more easily than the optical device illustrated in FIG. 1, and the overall size of the optical signal detecting apparatus can also be made more compact. However, such a structure can emit only a small portion of optical signals propagated through the core and the efficiency thereof is too small. Thus, the structure can only be used to monitor optical signals for checking only whether optical signals exist or not. That is, the optical device is not appropriate for use in an optical signal detecting apparatus which directly detects optical signals. The optical device is described in more detail in U.S. Pat. No. 5,042,897.
FIG. 2 is a cross-sectional view illustrating an optical receiver module using a conventional out-coupler, which is disclosed in Korean Patent Publication Gazette No. 0461154. The structure and function of the optical receiver module will be described briefly hereinafter.
Referring to FIG. 2, the optical receiver module includes an optical fiber 30, an optical fiber supporting unit 40 supporting the optical fiber 30 and including an out-coupler 42, and an optical receiver chip 50 receiving optical signals. The optical fiber 30 includes a core 32 and a cladding 34, and a portion of the core 32 is exposed so as to be coupled to the out-coupler 42. A grating 44 is formed in the out-coupler 42 and couples optical signals output from the core 32 to the optical receiver chip 50. The optical receiver chip 50 includes a light receiving portion 52 for focusing optical signals inside the optical receiver chip 50 and is supported on an optical receiver chip supporting substrate 60. An opening 62 is formed in the optical receiver portion 60 so that optical signals can be focused onto the receiving portion 52.
The optical receiver module has a structure in which optical signals are emitted through the exposed optical fiber core 32 and the out-coupler 42 in which the grating 44 are formed and the output optical signals are received by the optical receiver chip 50. The above described optical receiver module can achieve a coupling efficiency of 50% or greater. However, since an additional out-coupler is used on the outside, the size of the device becomes large and thus is inappropriate for high integration as the optical device illustrated in FIG. 1.